Purpose. To investigate the use of FDG-PET/CT before, during and after chemoradiotherapy (CRT) and diffusion-weighted magnetic resonance imaging (DW-MRI) before CRT for the prediction of pathological response (pCR) in rectal cancer patients. Material and methods. Twenty-two rectal cancer patients treated with long course CRT were included. An FDG-PET/CT was performed prior to the start of CRT, after 10 to 12 fractions of CRT and five weeks after the end of CRT. The tumor was delineated using a gradient based delineation method and the maximal standardized uptake values (SUVmax) were calculated. A DW-MRI was performed before start of CRT. Mean apparent diffusion coefficients (ADC) were determined. The Delta SUVmax during and after CRT and the initial ADC values were correlated to the histopathological findings after total mesorectal excision (TME). Results. Delta SUVmax during and after CRT significantly correlated with the pathological response to treatment (during CRT: Delta SUVmax = 59% +/- 12% for pCR vs. 25% +/- 27% if no pCR, p = 0.0036; post-CRT: 90% +/- 11 for pCR vs. 63% +/- 22 if no pCR p = 0.013). ROC curve analysis revealed an optimal threshold for Delta SUVmax of 40% during CRT and 76% after CRT. The initial ADC value was also significantly correlated with pCR (0.94 +/- 0.12 x 10(-3) mm(2)/s for pCR vs. 1.2 +/- 0.24 x 10(-3) mm(2)/s, p=0.002) and ROC curve analysis revealed an optimal threshold of 1.06 x 10(-3) mm(2)/s. Combining the provided Delta SUVmax thresholds during and after CRT increased specificity of the prediction (sensitivity 100% and specificity 94%). The combination of the thresholds for the initial ADC value and the Delta SUVmax during CRT increased specificity of the prediction to a similar level (sensitivity of 100% and specificity of 94%). Conclusions. The combination of the different time points and the different imaging modalities increased the specificity of the response assessment both during and after CRT.